The present invention relates generally to the production of hot rolled steel products and, more particularly, to a method and apparatus for producing hot strip material. The present invention further includes a novel, bi-level roller hearth furnace for heating discrete slabs received from two continuous slab casters. Still further, the invention relates to a method and apparatus for welding successive slabs together after exiting the two level furnace for subsequent continuous hot rolling to finished strip gauge.
It is well-known in the art to continuously roll hot strip by first continuously casting a strand of steel in one or more casters. The strand or strands are conventionally cut to a desired slab length downstream from the caster whereupon they enter a roller hearth or walking beam furnace for heating to a rolling temperature. The slabs leave the furnace and, depending upon the thickness, may then be reduced further in thickness in one or more roughing mills.
In order to provide a continuous length of metal to the hot strip mill, successive rough rolled slabs are end cropped and welded together, conventionally by butt welding the leading end of a trailing slab to the trailing end of a succeeding slab. A plurality of joined slabs are then continuously hot rolled to a desired strip thickness in a tandem hot strip finishing mill comprising a train of six or more mill stands. Heretofore, on-the-fly welding of slabs has caused problems due to poor welding strength, scale defects and/or thickness variations which produce later rolling defects in the strip. Welding on the fly also requires the creation of a buffer zone or extra space to accommodate the slow butt welding process relative to the speed of the mill. In addition, in order to run a conventional, continuous hot strip mill at high efficiencies, it is necessary to run more than one continuous slab caster. This is well-known in the art since the slab yield from one continuous caster is much less than the continuous hot strip mill rolling capacity.
When more than one continuous caster is employed to produce slabs for a hot strip mill, the handling of the slabs in the conventional pre-heat furnaces becomes a problem. Oftentimes, additional furnaces or much larger furnaces are required and/or transfer car systems must be employed in moving slabs laterally from a caster to an in-line or off-line furnace. Naturally, such larger capacity and/or additional furnaces and slab handling apparatus add considerable capital expense to the cost of the mill. Operating costs are also increased due to the additional fuel requirement in firing these extra and/or larger furnaces.
The present invention solves the problems heretofore encountered in the manufacture of continuous hot strip by providing a method and apparatus for heating slabs delivered from a plurality of continuous casters and for joining the slabs on the fly prior to rolling in a continuous hot strip finishing mill. Still further, the invention includes a furnace and an on-the-fly welding apparatus which provides nearly homogeneous temperature throughout the length of the welded transfer bar, precluding the need for so-called "zoom" or accelerated rolling in the hot strip mill. This leads to economies in mill motor sizing and electrical energy demands in running the hot strip mill.